The long-term objective of this project is to develop a halo orthosis that does not present the pin loosening problems associated with current halo orthoses. Presently, a patient being treated with a halo orthosis has a significant chance of experiencing a loose halo pin, leading to extreme patient discomfort, increased risk of infection, and possible loss of fixation of the skull and cervical spine. Additionally, in an effort to delay the onset of pin loosening, it's standard practice to over-tighten halo pins by at least 33% beyond what is needed to constrain the skull and cervical spine. This over-tightening increases patient discomfort and the chances of puncturing the skull with a halo pin. Pin loosening and the associated problems are especially prevalent in children since their skulls are more compliant. Some practioners use six or eight halo pins for children rather than the standard four pins in the anticipation of pin loosening. A halo ring that fully constrains the skull and cervical spine without pin loosening can be designed through a novel application of constraint theories that originated in the mechanisms research community. The root cause of pin loosening is the inability of current halo systems to adapt to small changes in skull and pin site geometry due to bone remodeling, stress relaxation, and pin site abrasion. Using constraint theories, an engineer can design a halo ring that adapts to these small changes much as highway bridges or aircraft are designed to adapt to small geometry changes due to thermal expansion. The specific aims of the proposed research are to 1) design and build a prototype of the adaptive halo orthosis, 2) design and build a skull simulator that enables testing of this and halo systems by measuring forces at the pin site and providing precisely adjustable pin site geometry, and 3) testing the pin loosening characteristics of the adaptive halo along with current halo systems.